13C Anisotropic Chemical Shifts and Spin-Rotation Constants in Nickel Tetracarbonyl and Iron Pentacarbonyl

The 13C nuclear spin-lattice relaxation time T1 was studied in liquid Ni(CO)4 and Fe(CO)5 as a function of temperature at resonance frequencies of 14, 30, and 61 MHz. Only two mechanisms contribute to the relaxation rate, anisotropic chemical shift and spin-rotation interaction. At 61 MHz both contributions are about equal for Ni(CO)4 in the entire temperature range studied, whereas for Fe(CO)5 relaxation through anisotropic chemical shift dominates at the lower temperatures. It is shown that in spherical molecules the close relationship between the spin-rotation interaction and the paramagnetic part of the chemical shift can be used to obtain the anisotropy of the chemical shift (Δ σ) and both spin-rotation constants from the relaxation data without use of adjustable parameters. The only additional information used is the relative isotropic chemical shift with respect to a reference compound (13CO in this case), for which the spin-rotation constant is known. The values found for δ σ in both complexes are within 10% of the value for gaseous CO. Measurements of the transverse relaxation time T2 are also discussed